CollectionView error : attempt to insert item 40 into section 0, but there are only 40 items in section 0 after the update

Question

The collection view will show list of 20 movies and if user scrolls to the last item, then it'll load another 20. At first I used reloadData() after appending another 20 items to the arrays of movies, And it works. But I believe the correct way to do it is to insertItems() to the collectionView instead of calling reloadData() every single time. After I implemented the insertItems(), Then the error occurs.

Here's the code related to the CollectionView :

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return moviesArray.count
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "MovieCell", for: indexPath) as! MovieCell
        let imageUrl = moviesArray[indexPath.row].posterPath!
        let url = URL(string: "https://image.tmdb.org/t/p/w500\(imageUrl)")!
        cell.moviePoster.load(url: url)
        cell.layer.cornerRadius = 10
        return cell
    }
    
    func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
        if indexPath.row == moviesArray.count - 1 {
            currentPage += 1
            dataManager.downloadAllMoviesJSON(page: currentPage)
        }
    }
    
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        dataManager.downloadMovieDetailJSON(id: moviesArray[indexPath.row].id) { (data) in
            DispatchQueue.main.async {
                self.performSegue(withIdentifier: "ToMovieDetail", sender: self)
            }
        }

And here's the code that handling the movies list loading and appending :

    func didGetMovies(dataManager: DataManager, movie: MovieData) {
        if currentPage > 1 {
            let lastInArray = moviesArray.count
            self.moviesArray.append(contentsOf: movie.results)
            let newLastInArray = moviesArray.count
            let indexPaths = Array(lastInArray...newLastInArray).map{IndexPath(item: $0, section: 0)}
            DispatchQueue.main.async {
                self.moviesCV.performBatchUpdates({
                    self.moviesCV.insertItems(at: indexPaths)
                }, completion: nil)
            }
        } else {
            moviesArray = movie.results
            DispatchQueue.main.async {
                self.moviesCV.reloadData()
            }
        }
    }

At first I didn't know that the insertItems() must be inside the performBatchUpdates() but even if I do so, The error still persists. It might be because I've implemented it improperly.

The error is always :

Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'attempt to insert item 40 into section 0, but there are only 40 items in section 0 after the update'

Perhaps anyone could kindly show the part that seems to be the issue?

Answer 1

The last index of an array is .count - 1. You have to use the ..< operator

let indexPaths = Array(lastInArray..<newLastInArray).map{IndexPath(item: $0, section: 0)}

and the performBatchUpdates block is not needed, it's only useful for simultaneous insert/delete/move operations

DispatchQueue.main.async { 
    self.moviesCV.insertItems(at: indexPaths) 
}